By the development of the moving picture communication such as a video conference, a video telephone and others, it is strongly desired to transmit the moving picture through the low bit rate digital channel. It is important for the technology to realize this requirement to encode the picture signal with high efficiency, and to transmit the encoded signal.
The high efficiency encoding system of the picture signal is classified into the intraframe-encoding system and the interframe-encoding system. The moving picture signal is considered a series of still pictures which are called picture frames of 25 through 30 frames per second. The intraframe-encoding system encoded by using only the data in the present picture frame, when any picture frame is encoded, and it is the interframe-encoding system that encodes by using the data of the present picture frame and the data of the previous frame.
When the receiver in this system decodes the encoded signal which is received from the transmitter, and reproduces the picture frame, the intraframe-encoding system can be decoded using only the data of encoded picture frame itself. On the other hand, it is necessary for the interframe-encoding system to decode the frame data using not only the present frame but also the preceeding picture frame.
Further, when an error of transmission occurs, only one frame is affected by the error of the transmission in the intraframe-encoding system. On the other hand, all succeeding frames are affected by the error of transmission in the interframe-encoding system. Accordingly, the normal decoding operation can not continue. To overcome the disadvantages of that interframe-encoding system, the picture frame is transmitted by the intraframe-encoding system when the error of transmission occurs, and then it is necessary to return from that system to the interframe-encoding.
The picture of the video conference, the video telephone and others are comparatively low in movement of a picture between successive frames. For those pictures, the interframe-encoding system is advantageous so that a high compression ratio is obtained. Accordingly, it is important that the system is quickly recovered when an error occurs.
A prior transmission system uses the fixed length transmission frame, in which the contents of the frame are identified by the header attached to the frame. But, in order to divide the variable length moving picture frame that is encoded by intraframe-encoding system or interframe-encoding system into the fixed length transmission frames, the aforementioned prior transmission system is not sufficient. It is necessary that the transmission system has the following: (1), the transmission efficiency for the moving picture signal is very high (compression ratio must be high); (2), If the error of transmission frame, the loss of transmission frame synchronization, and other errors occur, the system must recover in a short time. Further, it is necessary to be able to transmit other digital signals, for example, voice, still picture, data and others, together with the main moving picture signal. Accordingly, it is important for the proposed transmission system to be able to simply provide the multiplication of the signals.